Most absolute pressure sensors have a pressure reference chamber which is sealed by a diaphragm. Applied pressure deflects the diaphragm in an amount which depends upon the applied pressure. The applied pressure can therefore be measured by monitoring the deflection of the diaphragm. Deflection of the diaphragm can be monitored, for example, by measuring the capacitance of an air gap between the diaphragm and a reference plate. As is known to those skilled in the art, the capacitance depends upon the separation between the diaphragm and the reference plate. In the alternative, the stress on the diaphragm can be measured by a strain gauge coupled to the diaphragm. Either method can produce an output signal which corresponds to an applied pressure.
Diaphragm-type pressure sensors suffer from a number of disadvantages. Fabricating a reference chamber sealed by a diaphragm is more expensive than would be ideal. Further, diaphragm-type pressure sensors sense a mechanical signal. This signal may be caused to vary by external forces or vibrations. Therefore, such sensors must be mounted in such a manner that they are not subjected to forces which could distort the diaphragm or otherwise distort the output signal. These challenges have so-far interfered with the availability of inexpensive diaphragm-type pressure sensors which are sensitive, and which provide good long-term stability (e.g. better than a few mmHg per year for barometric pressure measurements).
It is well known that the thermal conductivity of a low-density gas is proportional to pressure. This principle has been applied successfully in thermal vacuum sensors. Such sensors typically operate at pressures below 100 Pa (approximately 0.001 atmospheres). At this low pressure the mean free path between molecular collisions is large. Thus, gas molecules can transfer heat directly from the source to the destination without colliding with other gas molecules en route. However, pressure sensors based upon variations in thermal conduction have not been considered practical for use in measuring pressures exceeding about 0.001 atmospheres. At higher pressures gas molecules have a shorter mean free path. Thus a gas molecule is likely to collide with another gas molecule before it has had a chance to travel much farther than the mean free path. This decreases the effectiveness of heat transfer by the gas molecules. At about one atmosphere (100 kPa) the mean free path of gas molecules decreases to about 0.1 micron. The thermal conductivity of a gas across a gap wider than a few microns is essentially independent of pressure as measured by current thermal pressure sensors.
There have been some attempts to increase the range of thermal pressure sensors by reducing the gap between a membrane area and unheated adjacent surfaces. Pressure sensors with an air gap as small as 0.3 μm have been reported. An example of such a narrow-gap device is described in Chou, B. C. S., Chen, C. N., Shie, J. S., Fabrication and Study of a Shallow-Gap Pirani Vacuum Sensor with a Linearly Measurable Atmospheric Pressure Range, Sensors and Materials, vol. 11, No.6 (1999) pp. 383-392. While providing a narrow gap can extend the pressure measurement range to several bars it would be desirable to be able to extend the pressure measurement range to even higher pressures. It is difficult with current semiconductor fabrication techniques to fabricate devices having air gaps much smaller than about 0.3 μm.
Scieferdecker et al., U.S. Pat. No. 5,597,957 describes another narrow-gap thermal pressure sensor. This device has a membrane suspended by webs of membrane material between a pair of mirrored walls. The walls are spaced apart from the membrane by distances of less than 5 μm. The Scieferdecker et al. device is undesirably complicated to manufacture and does not operate at pressures as high as would be desirable.
Despite the availability of diaphragm-type pressure sensors and narrow-gap thermal pressure sensors there remains a need for low-cost, stable-baseline pressure sensors that may be used for measuring manifold pressures in engines, barometric pressures, tire pressures, pressures in industrial processes and for use in other applications where higher gas pressures must be measured.